One of the most promising non-surgical artery-clearing procedures to emerge in recent years is angioplasty. The procedure calls for the use of a small inflatable balloon which dilates a narrowed artery and removes blockage. A catheter having an inflatable balloon secured to its distal end is advanced through an artery to a narrowed region. The balloon is then inflated with a fluid from an external source, causing the narrowed region of the artery to be expanded. The balloon is then inflated and withdrawn. However, the procedure creates arterial damage of its own which may cause the artery to narrow within three to six months in as many as 50% of successfully treated patients. If narrowing occurs, the patient may develop similar symptoms and risks again. Accordingly, the need has arisen to reduce angioplasty-induced lesions.
A technique which has shown promise for overcoming the problems of restenosis is the simultaneous application of heat and pressure to a plaque-narrowed region of an artery. In such techniques, a catheter having an inflatable balloon at its distal end is advanced to a narrowed region of an artery. The balloon is inflated, as in the case of conventional balloon angioplasty. However, in contrast to conventional balloon angioplasty, sufficient heat is applied through the wall of the balloon to fuse the surrounding tissue and thereby eliminate flaps which can later block an artery. One useful means of heating the surrounding tissue is by directing laser radiation through an optical fiber carried by the catheter and terminating within the balloon. The laser radiation is then directed through the balloon wall to cause heating of the surrounding tissue.
An earlier high power, helical, flexible diffusing tip was invented by Spears et al., U.S. Pat. No. 4,878,492, issued Nov. 7, 1989, and was used in the first clinical laser balloon angioplasty system disclosed in U.S. Pat. No. 4,799,479, also issued to the present inventor on Jan. 24, 1989. The disclosures of each of these patents are incorporated herein by reference.
Previously known designs of the diffusing tip, however, do not provide a cylindricalty symmetrical pattern of dispersion of fiberoptically-delivered laser energy. In addition, control of the axial propagation of radiation is difficult and unpredictable during fabrication of the diffusing tip, so that many diffusing tips are either inappropriately long or short. Moreover, the diffusing tip is relatively bulky, stiff, and prone to fracture.
Excessive radiation, particularly at the distal end of the diffusing tip, has necessitated the use of a highly reflective gold coating at the proximal and distal cone ends of the balloon surrounding the diffusing tip in order to prevent inadvertent heating of blood in contact with the balloon at these locations. But such an apparatus is only partially effective and expensive to fabricate.
Furthermore, the central channel and the guidewire within the central channel are exposed to a high dose laser energy. This has resulted in thermal shrinkage of the polyethylene central channel and bonding of the latter to the guidewire.
Thus, a diffusing tip which provides cylindrically symmetric dispersion of electromagnetic radiation at high power from the distal end of a fiberoptic, yet is flexible and resistant to damage at high power, has not been previously available.